The present invention generally relates to methods and systems for correcting offset of a head position signal, and more particularly to a method of correcting an offset of a head position signal which indicates a head position in a magnetic disk unit and is output from a position signal generating circuit, and a system for correcting the offset of the head position signal using this method.
A description will be given of a conventional method of reading a servo pattern signal which is recorded on servo cylinders of a magnetic disk using a magnetic head and generating a head position signal, by referring to FIGS. 1 and 2. FIG. 1 shows a servo pattern of the servo cylinders, and FIG. 2 shows an output signal waveform of a position signal generating circuit.
In order to match the position of a magnetic head on a track of the magnetic disk, the servo pattern signal is recorded on the servo cylinders of the magnetic disk as indicated by Odd and Even in FIG. 1. When the magnetic disk rotates in a direction X and the magnetic head traverses the servo pattern signal, a voltage is induced according to the Fleming's rule, thereby enabling the read out of the servo pattern signal. The voltage induced at the magnetic head is proportional to a magnetic flux density intersecting coils of the magnetic head. Accordingly, when the magnetic head is positioned at a point A corresponding to a center part of the signal Odd of the servo pattern, the induced voltage is a maximum with respect to the signal Odd and is zero with respect to the signal Even.
On the other hand, when the magnetic head is positioned at a point C, the induced voltage with respect to the signal Odd is zero and is a maximum with respect to the signal Even. In addition, when the magnetic head is positioned at a point B, the induced voltage with respect to the signals Odd and Even are the same but this induced voltage is relatively small.
When a difference of the induced voltages of the signals Odd and Even is obtained and the magnetic head is moved based on this difference, the difference of the induced voltages changes in a sinusoidal form as indicated by A in FIG. 2.
According to the conventional position signal generating circuit, the servo pattern of the servo cylinders is read in the above described manner, and a track center is described by a point where the difference of the induced voltages is zero. Hence, the difference of the induced voltage indicates the deviation of the magnetic head from the track center.
The use of data processing systems is becoming more and more popular, and there are increased demands on the magnetic disk units. That is, there are demands to reduce the size of the magnetic disk unit and to store data in the magnetic disks of the magnetic disk unit with a high density.
The position signal generating circuit described above includes an amplifier circuit for amplifying the induced voltages which are extremely small, an operational amplifier circuit for generating a signal which is dependent on the difference of the induced voltages, and a filter for eliminating noise and the like. But when the size of the magnetic disk unit is reduced by reducing the size of these circuits which form the position signal generating circuit, an offset is introduced in the position signal which is output from the position signal generating circuit due to an increase in temperature, aging and the like of the circuits.
When the position signal is offset as indicated by B in FIG. 2, the position of the magnetic head where the position signal becomes zero deviates from the true track center. As a result, although the true track centers are located at the positions A, B and C in FIG. 2, positions A', B' and C' are respectively detected as the track centers. In this case, the magnetic head reads or writes data at a position which is actually deviated from the true track center, and there are problems in that the magnetic head causes undesirable effects on adjacent tracks and that the induced voltage of the read data decreases and generates a large number of errors.
On the other hand, when the data is stored on the magnetic disk with a high density, the interval of the tracks formed on the magnetic disk, that is, the track pitch, becomes extremely small. For this reason, even if the offset value is within a conventionally tolerable range, there is a problem in that the offset value becomes too large because of the extremely small track pitch and undesirable effects are caused on the adjacent tracks.